Many radio frequency (RF) communications protocols rely upon amplitude modulation (AM) of an RF carrier signal to encode data therein. As RF communications protocols evolve, the required accuracy of this amplitude modulation continues to increase. For RF power amplifiers (PAs) used to amplify RF signals, the linearity of the RF PA determines its ability to accurately replicate the amplitude of an RF input signal to the radio frequency power amplifier in proportion to the gain of the radio frequency power amplifier. One way to increase the linearity of a radio frequency power amplifier is to increase a supply voltage provided thereto. Known in the industry as increasing a “headroom” of a radio frequency power amplifier, such an increase in linearity comes at the cost of efficiency, as additional power is expended due to an increase in the supply voltage. In order to increase the efficiency of radio frequency power amplifiers while simultaneously maintaining their linearity, many designers have turned to power supply modulation techniques such as envelope tracking. In general, envelope tracking maintains a minimum required voltage headroom for a radio frequency power amplifier in order to maintain the linearity of the radio frequency power amplifier over changes in the amplitude of an input signal. Because only the supply voltage necessary to maintain the linearity of a radio frequency power amplifier is used, envelope tracking results in significant increases in the efficiency of the radio frequency power amplifier when compared to a static supply voltage approach.
While envelope tracking often results in increased efficiency, modulating a supply voltage provided to a radio frequency power amplifier often results in changes in the gain response thereof. Specifically, the small signal gain of the radio frequency power amplifier will change depending on the provided supply voltage. Accordingly, the linearity of the radio frequency power amplifier will be significantly degraded when using envelope tracking power supply modulation, as discussed below.
FIG. 1 shows a basic configuration for a conventional RF PA circuitry 10 including power supply modulation circuitry 12. As shown in FIG. 1, the conventional RF PA circuitry 10 includes an amplifier stage 14 coupled to the power supply modulation circuitry 12. In operation, the amplifier stage 14 receives and amplifies an RF input signal RF_IN using the modulated power supply voltage M_VDD and the bias signal BIAS to provide an RF output signal RF_OUT. The power supply modulation circuitry 12 receives a power supply voltage VDD and provides the modulated power supply voltage M_VDD to the amplifier stage 14. Bias circuitry 16 provides the bias signal BIAS to the amplifier stage 14, which sets one or more operating parameters of the amplifier stage 14.
FIG. 2 shows the gain response of the conventional RF PA circuitry 10 for a variety of different power supply voltages VDD that may be provided by the power supply modulation circuitry 12 at a given time. In FIG. 1, the various power supply voltages VDD increase between VDD1 and VDD5. Notably, the small signal gain at various power supply voltages VDD diverges significantly between VDD1 and VDD4, which results in disruptions in the linearity of the conventional radio frequency power amplifier, also known as AM to AM distortion. Further, while the small signal gain between VDD4 and VDD5 remains relatively constant, the point at which gain compression and/or expansion begins varies significantly between these supply voltages.
In addition to the shortcomings of power supply modulation with respect to AM to AM distortion, power supply modulation can also result in significant changes to the phase of an RF signal provided to a radio frequency power amplifier. As shown in FIG. 3, the conventional RF PA circuitry 10 includes a number of parasitic capacitances C_P. As the modulated supply voltage M_VDD provided to the conventional RF PA circuitry 10 changes, the capacitive response of the parasitic capacitances also changes, resulting in changes to the phase of the RF signal RF_IN as it is passed through the conventional RF PA circuitry 10. FIG. 4 shows changes in the capacitance of the amplifier stage 14 for different supply voltages VDD provided thereto. As shown in FIG. 4, the input capacitance remains relatively constant for a particular input power range of the amplifier stage 14, then decreases sharply. While the variable capacitance of the amplifier stage 14 can often be compensated for in static supply voltage systems, the point at which the input capacitance decreases changes significantly with different supply voltages provided to the amplifier stage 14, making such compensation extremely difficult. As discussed above, these changes in the capacitance of the amplifier stage 14 result in significant changes to the phase of the RF signal RF_IN, also known as phase modulation (PM) to PM distortion and or AM to PM distortion.
Accordingly, there is a present need for RF PA circuitry configured to maintain a constant gain and phase response over a variety of power supply voltages such that the RF PA circuitry maintains low distortion levels when used with power supply modulation techniques.